The present invention relates to a control system for a plant, and more particularly to a control system controlling a plant with a response specifying type controller based on a sliding mode control theory which is one of robust control theories.
There has been known a sliding mode controller for controlling a plant according to a sliding mode control which is one of response specifying type controls (Japanese Patent Laid-open No. 2000-110636, for example). Specifically, the sliding mode controller shown in this publication controls an internal combustion engine. In the sliding mode control, it is possible to specify (change) a damping characteristic of a deviation between an output of the plant (controlled object) and a control target value. Therefore, such control is called as xe2x80x9cresponse specifying type controlxe2x80x9d. Other than the sliding mode control, a back stepping control is also known as a response specifying type control. In the response specifying type control, a control input to a plant is calculated using a switching function which is defined as a linear function of a deviation between a control target value and an output of the plant, and a damping characteristic of the deviation can be changed by changing the switching function.
In the above response specifying type control, there are two control modes. One is a reaching mode in which a value of the switching function is converged to xe2x80x9c0xe2x80x9d. The other is a sliding mode in which a control deviation is converged to xe2x80x9c0xe2x80x9d maintaining the condition where the switching function value equals xe2x80x9c0xe2x80x9d. The sliding mode is a robust control mode against disturbance or a modeling error (a difference between the characteristics of the actual controlled object and the characteristics of the controlled object model). However, in the reaching mode, the control may easily be affected by a change in the dynamic characteristics of the controlled object or disturbance. Accordingly, in the reaching mode, a change in the dynamic characteristics of the controlled object or disturbance may sometimes cause a condition where an overshoot of the plant output with respect to the control target value is prone to occur.
It is an object of the present invention to provide a control system for a plant which is capable of positively preventing an overshoot in the converging process of the plant output to make the plant output converge smoothly to the control target value.
To achieve the above object, the present invention provides a control system for a plant. The control system includes a response specifying type controller (21) for controlling the plant with a response specifying type control so that an output (DTH, DPACT) of the plant coincide with a control target value (DTHR, DPCMD). The response specifying type controller (21) includes switching function value calculating means, reaching mode input calculating means, and correcting means. The switching function value calculating means calculates a value ("sgr") of a switching function which is defined as a linear function of a deviation (e) between the output (DTH, DPACT) of the plant and the control target value (DTHR, DPCMD). The reaching mode input calculating means calculates a reaching mode input (Urch, Uadp) that contributes to placing a deviation state quantity (e(k), e(kxe2x88x921)) which is defined based on the deviation (e), onto a switching straight line on which the value ("sgr") of the switching function becomes zero. The correcting means corrects the reaching mode input (Urch, Uadp) according to the value ("sgr") of the switching function. A control input (Usl) from the response specifying type controller (21) to the plant includes the reaching mode input (Urch, Uadp).
With this configuration, the reaching mode input that contributes to placing the deviation state quantity onto the switching straight line is calculated, and the calculated reaching mode input is corrected according to the value of the switching function. This correction makes it possible to prevent an overshoot in the converging process of the plant output to make the plant output converge smoothly to the control target value.
Preferably, the reaching mode input calculating means calculates a reaching law input (Urch) by accumulating a value which is obtained by multiplying an amount of change ("sgr"(n)xe2x88x92"sgr"(nxe2x88x921)) in the value of the switching function with a first gain (xe2x88x92F/b1) (Eq. (9b)), and the correcting means changes the first gain (xe2x88x92F/b1) according to the value ("sgr") of the switching function.
With this configuration, the reaching law input is calculated by accumulating a value which is obtained by multiplying an amount of change in the value of the switching function with the first gain. This calculation method of the reaching law input prevents abrupt change in the reaching law input when the first gain is corrected by the correcting means, to thereby prevent reduction of controllability upon changing the first gain.
Preferably, the reaching mode input calculating means calculates an adaptive law input (Uadp) by accumulating a value which is obtained by multiplying the value ("sgr") of the switching function with a second gain (xe2x88x92Gxcex94T2/b1) (Eq. (10b)), and the correcting means changes the second gain (xe2x88x92Gxcex94T2/b1) according to the value ("sgr") of the switching function.
With this configuration, the adaptive law input is calculated by accumulating a value which is obtained by multiplying the value of the switching function with the second gain. This calculation method of the adaptive law input prevents abrupt change in the adaptive law input when the second gain is corrected by the correcting means, to thereby prevent reduction of controllability upon changing the second gain.
Preferably, the reaching mode input calculating means includes limit means (S251-S255, S271-S275) for limiting the value ("sgr") of the switching function within a predetermined range (xe2x88x92XSGMSL, XSGMSL), and calculates the reaching mode input (Urch, Uadp) using the switching function value (SGMS) limited by the limit means.
With this configuration, the value of the switching function is limited within the predetermined range, and the reaching mode input is calculated using the limited switching function value. This makes it possible to prevent an overshoot when the control target value abruptly changes.
Preferably, the correcting means reduces an absolute value of the first gain (xe2x88x92F/b1) or the second gain (xe2x88x92Gxcex94T2/b1) as an absolute value (|"sgr"|) of the switching function increases (FIG. 13A).
Preferably, the control system further includes identifying means (22) for identifying a model parameter vector (xcex8) of a controlled object model which is obtained by modeling the plant, and the reaching mode input calculating means calculates the reaching mode input (Urch, Uadp) using an element (b1) of the model parameter vector (xcex8).
Preferably, the plant includes a throttle valve actuating device (10) having a throttle valve (3) of an internal combustion engine (1) and actuating means (6) for actuating the throttle valve (3), and the response specifying type controller calculates a parameter (DUT) for determining a control input to be applied to the throttle valve actuating device (10) to make an opening (TH) of the throttle valve coincide with a target opening (THR).
With this configuration, an opening of the throttle valve is controlled to coincide with the target opening with the control input including the reaching mode input. Accordingly, it is possible to prevent an overshoot of the throttle valve opening to the target opening, to thereby make the throttle valve opening converge smoothly to the target opening.
Further the present invention provides another control system for a plant. The control system includes a response specifying type controller (21) for controlling the plant with a response specifying type control so that an output (DTH, DPACT) of the plant coincide with a control target value (DTHR, DPCMD). The response specifying type controller (21) includes switching function value calculating means, reaching mode input calculating means, and correcting means. The switching function value calculating means calculates a value ("sgr") of a switching function which is defined as a linear function of a deviation (e) between the output (DTH, DPACT) of the plant and the control target value (DTHR, DPCMD). The reaching mode input calculating means calculates a reaching mode input (Urch, Uadp) that contributes to placing a deviation state quantity (e(k), e(kxe2x88x921)) which is defined based on the deviation (e), onto a switching straight line on which the value ("sgr") of the switching function becomes zero. The correcting means corrects the reaching mode input (Urch, Uadp) according to the deviation (e) (FIG. 13B). A control input (Usl) from the response specifying type controller (21) to the plant includes the reaching mode input (Urch, Uadp).
With this configuration, the reaching mode input is corrected according to the deviation between the output of the plant and the control target value. This correction makes it possible to prevent an overshoot in the converging process of the plant output when the control target value abruptly changes, to thereby make the plant output converge smoothly to the control target value.
Preferably, the correcting means reduces an absolute value of the reaching mode input (Urch, Uadp) as an absolute value of the deviation (e) increases.
Further the present invention provides another control system for a plant. The control system includes a response specifying type controller (21) for controlling the plant with a response specifying type control so that an output (DTH, DPACT) of the plant coincide with a control target value (DTHR, DPCMD). The response specifying type controller (21) includes switching function value calculating means, reaching mode input calculating means, and correcting means. The switching function value calculating means calculates a value ("sgr") of a switching function which is defined as a linear function of a deviation (e) between the output (DTH, DPACT) of the plant and the control target value (DTHR, DPCMD). The reaching mode input calculating means calculates a reaching mode input (Urch, Uadp) that contributes to placing a deviation state quantity (e(k), e(kxe2x88x921)) which is defined based on the deviation (e), onto a switching straight line on which the value ("sgr") of the switching function becomes zero. The correcting means corrects the reaching mode input (Urch, Uadp) according to the output (DTH) of the plant (FIG. 13C). A control input (Usl) from the response specifying type controller (21) to the plant includes the reaching mode input (Urch, Uadp).
With this configuration, the reaching mode input is corrected according to the output of the plant. This correction makes it possible to set the reaching mode input to a value which is suitable for the dynamic characteristics of the plant that change depending on the output of the plant, to thereby improve controllability of the control system.